1. Field of the Invention
The present invention relates to amorphous alloy strips having a large thickness and a method for producing the same, more particularly to amorphous alloy strips having a large thickness produced by quenching and solidifying molten metal or alloy on a movable cooling substrate and a method for the same.
2. Description of the Related Art
It is well known to use a melt spin process to continuously produce amorphous strips from molten metal or alloy. In the melt spin process, molten metal is deposited onto a cooling substrate, e.g., the surface of annular chill roll, through a nozzle or nozzles. The molten metal is quenched and solidified by the cooling substrate, resulting in a continuous metal strip or wire.
In the melt spin process, the cooling rate is so high that, if the composition is suitably selected, an amorphous metal or alloy having substantially the same structure as the molten metal can be obtained. An amorphous metal or alloy has unique properties valuable for practical applications.
There are, however, some difficulties in obtaining wide strips. Important factors in the production of an amorphous metal or alloy are the shape of the nozzle, the relative arrangement of the nozzle and cooling substrate, the ejecting pressure of the molten metal through the nozzle, and the moving rate of the cooling substrate. To increase the width of the strip, one must meet severe conditions for each of the above.
A continuous casting method for a metallic amorphous strip and an apparatus for producing a wide strip are disclosed in Japanese Unexamined Patent Publication (Kokai) No. 53-53525. The method includes the steps of directing a slotted nozzle having a rectangular opening to a cooling substrate (roll or belt) with a gap of from about 0.03 to about 1 mm therebetween, advancing the cooling substrate at a speed to provide a peripheral velocity of from about 100 to about 2000 meters per minute, and ejecting molten metal to the chill surface of the cooling substrate through the slotted nozzle. The molten metal is quenched in contact with the chill surface at a rapid quenching rate and solidifies into a continuous amorphous metal strip. In this method, there is no limit on the width of the amorphous metal strip, in principle.
Restrictions on the cooling rate also make it difficult to obtain a thick strip. The problem of thickness of increasing the thickness of the strip has not been solved up until now. This limit on the thickness of the strip applies not only to amorphous metal requiring severe cooling conditions, but also to crystalline metal not requiring the same. The principal method adoptable to try to form a metal strip having a large thickness in the conventional continuous molten metal quenching process is to increase the advancing length of the puddle formed on the cooling substrate with respect to the advancing speed of the cooling substrate. In actual production of an amorphous metal strip, any one of the following means or combinations thereof may be considered to achieve this increase: The means are
1. To enlarge the width of the nozzle opening
2. To increase the forcing pressure
3. To increase the gap between the nozzle and the chill surface
4. To decrease the advancing speed of the cooling substrate
The present inventors experiments to produce an amorphous metal strip having a large thickness by using the above four means, but could not obtain good results. They found that there is a limit on thickness due to the type of metal or alloy and the material of the cooling substrate and that an unreasonable increase in thickness leads to an undesired shape and deterioration of the strip. Excessive molten metal, specifically, adheres to the nozzle and solidifies thereon. The solidified metal, which contacts the advancing chill surface, leads to nozzle breakage. Also, when a thick strip is produced by the above four means, the free surface of the metal strip is exposed to the atmosphere for a longer time, resulting in an undesired appearance, such as a rough surface, furrows, and coloring. Generation of such phenomena, in the case of an amorphous alloy, means also that crystal is formed on the surface layer, even if the crystal cannot be detected by X-ray diffraction. This reduces the ductility, the magnetic properties such as coercive force and core loss , and other properties of the amorphous alloy.
IEEE Trans., May 18 (1982) page 1385, discloses that if the strip thickness at which the coercive force begins to increase is defined as the critical strip thickness at which crystallization commences, the greatest critical strip thickness shown by an Fe-Si-B system alloy is 42 .mu.m of Fe.sub.76 -B.sub.10 -Si.sub.10. According to investigations by the present inventors, with Fe.sub.80.5 Si.sub.6.5 B.sub.12 C.sub.1 of a width of 25 mm, the critical strip thickness is 32 .mu.m. Further U.S. Pat. No. 4,331,739 discloses F.sub.40 Ni.sub.40 P.sub.14 B.sub.6 of a width of 5 cm, a thickness of 0.05 mm (50 .mu.m), and isotropic tensile properties.
Recently, an Fe base alloy strip having a width of 25.4 mm and a thickness of 82 .mu.m was reported (Journal of Applied Physics vol. 5, No. 6 (1984) P. 1787). According to the report, however, this alloy strip, of Fe.sub.80 B.sub.14.5 Si.sub.3.5 C.sub.2 showed the existence of 5% or less crystals under an X-ray diffraction test. As a consequence, the alloy strip as cast shows considerable brittleness. The fracture strain by bending stress of an 82 .mu.m thick Fe.sub.80 B.sub.14.5 Si.sub.3.5 C.sub.2 alloy is 0.006. The fracture strain .epsilon..sub.f is usually represented by the equation .epsilon..sub.f =t/(2r-t), wherein t is the strip thickness and r is the bending radius.
The more amorphous the alloy, the greater the fracture strain. A substantially amorphous alloy has a crystallization ratio of 1% or less as cast. The crystallization ratio is defined as follows: EQU Fc=(I-Io)/Ic
wherein I is the diffraction intensity on a specified crystal face for example (110) face of a sample of a strip as cast, Io is the diffraction intensity on the same crystal face of a standard amorphous sample, and Ic is the diffraction intensity on the same crystal face upon complete crystallization.